1. Field of the Invention
The present invention relates to a power converter that outputs a DC voltage from an AC voltage by on-off controlling a switching element, the DC voltage being generated with energy stored in a an inductor. In particular, the invention relates to a power converter operated as a step-up or step-down chopper.
2. Description of the Related Art
FIG. 5 shows an example of a conventional power converter using a switching element in which a power factor correction circuit is provided in the output side of a rectifying circuit. In the power converter of FIG. 5, an AC power supply 1 supplies a DC voltage to a load 20 through a switching power supply 10.
The switching power supply 10 comprises a noise filter 2 consisting of capacitors 2a, 2b, 2c and a common mode choke coil 3, a rectifying circuit 4 consisting of diodes 4a, 4b, 4c, 4d, an inductor 5 for correcting a power factor disposed in the side of an output line of the rectifying circuit 4, a switching element 6 on-off controlled by a PWM control circuit 6a, and a smoothing circuit consisting of a diode 7 and a capacitor 8. The noise filter 2 is provided for reducing the conduction noise from the power converter to the AC power supply 1. A MOSFET is used for the switching element 6.
Operation of the power converter 10 is described below.
When the switching element 6 is turned on, the inductor 5 is supplied with an electric current and stores energy. Then, when the switching element 6 is turned off, the stored energy is transferred to the load 20. In this stage, the control circuit 6a controls pulse widths of on-off operation of the switching element 6 to make an input current from the AC power supply to be a sinusoidal waveform for correcting the power factor.
Switching operation of a MOSFET used in a rectifier circuit or a inverter circuit composing a power converter such as a PWM inverter is carried out based on driving signals that are pulse wave modulated (PWM) with a carrier frequency in the range from several kHz to ten and several kHz. It is known that the switching operation generates common mode noises containing components at frequencies higher than several tens of kHz between the main circuit conductor and the ground in the power converter.
In order to reduce the conduction noise generated in this type of power factor correction circuit, Patent Document 1 discloses a circuit construction in which a choke coil is divided into two coils each having an equal half number of windings wound on a single core in the opposite polarity, and inserted and connected to the both ends of an output line of the rectifying circuit. In this circuit construction, the high frequency noises such as ripple current generated at the voltage supplying side line and the negative side line from the power factor correction circuit by on-off operation of the switching element appear in the opposite phases, cancelling normal mode noise generated between the voltage supplying side line and the negative side line of the rectifying circuit and diminishing the noise level.
In this type of power converter, the current in the inductor 5 provided for suppressing the normal mode noise has an AC waveform of a superposition with a low frequency component of a sinusoidal waveform and high frequency components due to on-off operation of the switching element 5. The instantaneous current at the peak in the low frequency component is substantially large. A magnetic flux density in the magnetic core at an average current value needs to be so small that the core does not saturate at the moment of this peak. Accordingly, the core needs to be designed in a large size. Further, in a devise that is to permit an overload in a short duration, the core size is necessarily still larger, entailing a problem of enlarged size of an overall power converter.
In the case of connection to an external power source such as a commercial power line, a common mode choke coil 3 is required in a nose filter 2 as shown in FIG. 5. However, the conventional device as disclosed in Japanese Unexamined Patent Application Publication No. 5-191976, FIG. 1) restrains only normal mode noise and cannot suppress common mode noise. Accordingly, a means for suppressing the common mode noise must be provided in addition to the inductor 5 for power factor correction. This is a large obstacle against minimization of a power converter.